In recent years, optical computing techniques have been developed for applications in the oil and gas industry in the form of optical sensors in downhole or surface equipment to evaluate a variety of fluid properties. In general, an optical computing device is a device configured to receive an input of electromagnetic radiation from a sample and produce an output of electromagnetic radiation from a processing element, also referred to as an optical element, wherein the output reflects the measured intensity of the electromagnetic radiation. The optical element may be, for example, an Integrated Computational Element, or ICE. One type of an ICE is an optical thin film optical interference device, also known as a multivariate optical element (“MOE”).
Fundamentally, optical computing devices utilize the optical elements to perform calculations, as opposed to the hardwired circuits of conventional electronic processors. When light from a light source interacts with a substance, unique physical and chemical information about the substance is encoded in the electromagnetic radiation that is reflected from, transmitted through, or radiated from the sample. Thus, the optical computing device, through use of the ICE and one or more detectors, is capable of extracting the information of one or multiple characteristics/analytes within a substance and converting that information into a detectable output signal reflecting the overall properties of a sample. Such characteristics may include, for example, the presence of certain elements, compositions, fluid phases, etc. existing within the substance.
Historically, the thin-film MOEs have been designed and fabricated using alternating layers of high-index and low-index materials deposited on a substrate. Once the materials have been deposited on the substrate, however, the transmission/reflection/absorption functions of the MOE are fixed due to the fundamental nature of the design and fabrication process. Therefore, once the film stack has been deposited, its spectral properties cannot be changed.